Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to a method and apparatus of classifying x-ray energy into discrete levels.
Typically, in radiographic imaging systems, such as x-ray and computed tomography (CT), an x-ray source emits x-rays toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” may be interchangeably used to describe anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-rays. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image. In CT imaging systems, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray detectors also typically include a collimator for collimating x-ray beams received at the detector.
In some x-ray or CT imaging systems, the detector includes scintillator detector cells for converting x-rays to light energy and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data acquisition and processing system for image reconstruction.
In other x-ray or CT imaging systems, the detector includes direct conversion detector cells for converting x-rays directly to electrical signals indicative of the amount of energy in the x-rays. The direct conversion detector may be operated in a photon counting mode configured to provide information regarding each separate x-ray detected or in a current-integration mode configured to integrate the energy received from x-rays over a period of time.
A drawback of such scintillator-operated or direct-conversion-operated current-integrating detectors, however, can be their inability to provide data or feedback as to the number and/or energy of photons detected as their signal outputs are a mix of the number of and the energy of the incident photons. That is, for current integrating detectors, either scintillator-based or direct-conversion based, the signals emitted during readout are a function of the number of x-rays impinged over a period of time as well as the energy level of the impinging x-rays. As x-ray imaging systems almost exclusively involves polychromatic (multi-energy) x-ray sources, the combinations of number and energy of impinging broad-spectrum photons is variable. This variability depends heavily on the x-ray source used and the type and thickness of material examined. Under the charge or current integration operation mode, the detector is not capable of discriminating between the energy level or the photon count from individual photons when two or more photons are detected of different x-ray photon energies. For example, two integration-based detector cells may produce an equivalent output from their respective photodiodes, although the number and energy of the photons impinging on each detector may be substantively different.
For providing data or feedback as to the number and energy of photons detected at the detector, x-ray or CT imaging systems may use the direct conversion detector in a photon counting mode that includes detector cells capable of providing data as to the number and energy of the photons detected. Alternatively, the x-ray or CT imaging systems may use a fast scintillator detector that includes detector cells capable of providing data very quickly after the photon is received that indicates the energy of the detected photon prior to receiving another photon. In addition, a data acquisition system (DAS) coupled to the detector is provided to sort the photons into energy storage bins based on their detected energy. Conventional photon-counting imaging systems often employ multiple hardware energy storage bins for classifying the photons. As the number of energy bins increases, the amount of spectral information about the incoming x-ray beam also increases. That is, the ability to classify photons into, for example, two different energy bins provides more detailed information than classifying the same photons into only one energy bin (such as all photons having an energy above a base noise level). Likewise, classifying the photons into five different energy bins provides more detailed information than classifying the same photons into the two energy bins in the previous example.
However, as the number of energy bins increases, so does the cost and complexity of the imaging system. In addition, to add additional energy bins to an existing system in order to increase its bin count or to replace the entire DAS with another DAS having more energy bins is also cost-prohibitive.
Therefore, it would be desirable to design a system and method for classifying photons into a number of energy classifications or bins greater than the number of hardware energy storage bins in the DAS of an imaging system.